1. Field of the Invention
The invention relates to a sheet transfer drum in a multi-color sheet-fed rotary printing press. The sheet transfer drum is of the type which has a central drum body rotatably mounted about a drum axis in the machine frame and with at least one gripper system extending across the sheet and with jacket segments, which are disposed on a respective flat part of the drum body and which form a sheet-carrying jacket surface which is essentially closed during operation.
2. Description of the Related Art
A sheet transfer drum with those features has become known heretofore from German Patent DD 125 394 (East Germany). That prior art reference describes a sheet transfer drum disposed between two printing units of a multi-color sheet-fed rotary printing press for recto-verso printing. The transfer drum is used in that case as a turning drum, which guides the sheet that has been printed on one side and turned to a following printing unit. There is illustrated an enlarged sheet transfer drum with two gripper systems, which are approximately diametral to the drum axis and which grip a sheet at its leading edge one after the other. The sheet thereby comes to lie against the jacket surface (peripheral surface) of the sheet transfer drum. The contact surface on the jacket of the sheet transfer drum upon which the sheet comes to lie is formed of segment covers, which are detachable and therefore removable or exchangeable for format adaptation. The segment covers each are attachable to a flat part of the drum body between the gripper systems and they extend in one part diagonally over the width of the sheet.
In order to avoid smearing on the sheet transfer drums due to contact of the fresh ink with the jacket surface of the sheet transfer drum, there are provided, according to German Patent DE 35 35 621 C2, small spurs on the jacket surface of that sheet transfer drum which are mounted on guide rods. The spurs support the sheet in unprinted areas and protrude radially from the jacket surface. When not in use, the spurs are brought into a position which is radially pulled-back relative to the jacket surface. Instead of single spurs it is also known from that reference to provide stiff, interconnectable jacket segments. Those jacket segments can be moved in a radially pulled-back position and their format can be adapted.
In order to avoid smearing of the fresh ink in the area of a sheet transfer drum between printing units of a multi-color sheet-fed rotary printing machine for recto-verso printing, hollow sheet guide bars are known from German Patent DD 57 345 (East Germany), from which sheet-supporting air flows out through bores. The sheet is thus prevented from touching the guide bars disposed below the sheet transfer drum.